Full Throttle Hesitation :(

[peterako]

Hi all,

After a couple of days reading through the archives here I am not satisfied I've found any pointers as to my problem.

And I'm pretty sure someone here must have experienced and cure this.

My engine is recently rebuilt. And probably for the first time ever has good compression on all cyliners.

Engine is at approx 10 degrees static.

On Dell'Ortos.

Lumenition Optronic Ignition.

All valve clearances withing range except on exhaust 1 thou over and one inlet 2 thou over. (Will be reshimming in another couple of hundred miles)

My problem:

I get either hesitation at 4000 rpm starting a 100 to 200 before 4000 and ending about the same after.

If I back off even the smallest amount from full throttle the hesitation does not occur.

Eg.

Accellerate at full throttle to 3500 (no hesitation), stay on full throttle and at 4000 +/- 100 or so - hesitation, above that hesitation gone again.

Accellerate at full throttle to 3500 (no hesitation), back off full throttle at 3500 and then back on at 4200 or so - NO hesitation etc.

Same problem exists at 12 degrees static. (Onlly difference was less responsiveness)

So am beginning to wonder if it's not timing but something else.

I know that I rejetted idle on the Dell'Ortos leaner before this rebuild.
There was no hesitation at any part of the rev range at that stage. But then I had less compression.

Any pointers where to start??

Thanks in advance,
Peter

[worzel]

Hi

Just a simple trick- when you reach hesitation point pull out the choke a bit- if it cures the problem you're underfuelling- if it goes worse you're overfuelling. Not exactly scientific/totally accurate but quick and easy to try.

Regards

John

[peterako]

Excellent idea John!

Will do.

Thank you. Peter

[AHM]

Check float levels

[peterako]

Hi John and AHM,

Tried the choke test. No difference.

So started thinking about float levels affecting the Emulsion Tubes.

The previous day I had, I thought, re-timed more accurately....more on that later.

So, last night I readjusted the floats to 15mm (14.96mm) with my nice digital Vernier.

Re-adjusted the Idle speeds with the idle screws and rebalanced.

Car was MUCH better today, though the hesitation had moved up to 4300-4500.....Hmmm.

But, car drove well.

Went down to my local friendly mechanice (Paul Cobban) and we checked that the total advance at high revs was no more than 28 and checked idle advance too.

28 and 10 respecitvely so all seemed fine till...

Which mark on the case are we using' was asked. It was timed against the 10 mark so....we reset to the proper mark (I know it's not totally accurate).

While doing this I we noticed that Carb 1 (3 and 4) is weeping....dang floats or valve.

Getting there I hope.

Many thanks for your help thus far. Will check the fuel valve this evening.

Will also try and work out where real TDC is.

Seems with all this new Power (new rebuild) comes new adjustments. But it will be worth it.

The car was lovely with the (cough) 20 degree Static and 38 degree max advance

Peter

[AHM]

Next try.
A new fuel filter if you have one. If you don't have one fot one!
Clean out the crud in the bottom of the fuel pump.

[peterako]

AMH:

I have a filter......but it's been sitting on my centre console for a few years

Short Update:

After last night's fettling -

- Check floats in 'flooding carb' (There is no flooding)
- Check that float valve is closing tight (It was)
- Do a bit with timing etc.

No time for a drive, but idle was nice enough.

This moring I started the car, with great difficulty....it did NOT like having any choke (Dell'Ortos and 3 C this morning brrr).

Normally it woudl start first turn from cold....

Idle and pick-up not great too....

To add insult to injury....could not keep up with a 1.6 ltr Toyota Corolla Saloon while overtaking a lorry

Was on the way to my friendly Classic's Garage to check the timing I'd set last night.

It was 10 degrees 'static', more or less same as yesterday when leaving there. But now starting, idle, accelleration etc were horrible.

So....plan B....old fashioned method:

Step one - reset the timing....without a gun....
Step two look at carbs again.

Step 1:

Start car and get idle to approx right revs.
Turn distributor clockwise/anticlockwise (all the while ensuring that idle was adjusted to correct speed as it may rise/fall) and choose best idling spot. (I actually backed off a tiny bit).
Go for a spirtited drive and make sure no pinking.

The difference was immense. Roughly twice the accelleration. Almost no hesitation anywhere in range in 1 - 4th gears. Indeed much improved over my initial problems.

Step 2:

To look at carbs....

Now I'm beginning to thing that the problem in fueling, if any, is due to a bodge job I may ( ) have done many years ago when overhauling the carbs.
I broke the accellerator pump bar on one carb and had to refit with it shortened.
I'm pretty sure that carb is dumping too much fuel in too early. With the Dell's even the gentlest throttle depression seems to inject fuel.
I have a spare bar and will take the carbs off soon to change it.

So....we put the light back on the crank pully:

Back at my 20 degrees static which I had when the car was going nicely yesterday.

I have not checked where real TDC is but will. It is possible that the pully mark may be out by 10 degrees. If this is the case then I hope the we have the cams right Nothing is touching/banging. Car is going like the 'clappers'.....

Thanks again for all your help.

Take care,
Peter

[peterako]

Quick update:

Fitted a new Distributor (only one I could get at a reasonable price was from AccuSpark in the UK) on the weekend.

Engine running much more smoothly now. Idling well and plenty of low down torque, which I never seemed to have before at these levels.

I had begun (in the back of my mind) to suspect that the original distributor was well worn for a short while now.
It may have been advancing much too quickly.
It seemed to be adding 18 degrees of advance at the top which seemed to be before 3500 revs.

The new distributor claims 16 degrees which comes in at full at 5000 or so. (Have not checked)
This is close to the original spec. distributors.

4000 rpm full throttle hesitation is now minimal (possible butterfly issue).

Also got a mamometer on the carb bypasses before and after the distributor change.

Before - Cyl 4 pulling a lot more than 2 and 3 which were equal and cyl 1 pulling a lot less.
After - Cyl 4 pulling slightly more than 1, 2 and 3 which are now equal.

Suspect a lead problem on top of the old distributor problem as have also changed the leads with the new distributor.

The cylinder 4 difference may be vacuum (brake servo) related. Will check when I get a chance.

My old distributor was running a Lumenition Optronic, the new the AccuSpark system.

Might pick up another distributor from AccuSpark and fit the Optronic to it at some stage.

Very happy with Tony at AccuSpark for all the help he provided in my purchase as for some reason I could not complete the transaction on eBay.co.uk.

NOW I'll start to fine tune the carbs again

Thanks for all your help guys!

Peter